1. Field of the Invention
The present invention relates to a microcrystal silicon film used as a component of a solar cell and a thin-film transistor, and to a manufacturing method of such a microcrystal silicon film.
2. Description of the Related Art
Microcrystal silicon is known as a material which exhibits intermediate properties between amorphous silicon and single crystal silicon. As is known from Japanese Examined Patent Publication No. Hei. 3-8102 and Japanese Unexamined Patent Publication No. Sho. 57-67020, a plasma CVD method is known as a manufacturing method of a microcrystal silicon film. In this method, a microcrystal silicon film is deposited on a substrate by decomposing a mixed gas of a silane gas and a hydrogen gas by glow discharge. This method is characterized in that the mixed gas that is supplied to a reaction space for forming a film should be composed such that the amount of hydrogen gas is tens to hundreds of times larger than the amount of silane gas, and in that glow discharge is caused by inputting electric power at a high density. Further, if a diborane gas, a phosphine gas, or the like is added to the above mixed gas for the purpose of valence electron control, doping is effected so efficiently that there can be obtained a high electric conductivity which cannot be attained by an amorphous silicon film. For this reason, a microcrystal silicon film is frequently used as a valence-electron-controlled doped layer, i.e., a p-type or n-type layer to constitute a photocell or a thin-film transistor.
In manufacture of a microcrystal silicon film in which a silane material gas is diluted, the film forming rate is substantially determined by the silane gas supply amount and is lower than that of an amorphous silicon film. The film forming rate of a microcrystal silicon film is approximately in a range of 0.01-0.1 nm/s. A film forming rate lower than this range is not practical, whereas a microcrystal silicon film is not formed at a film forming rate higher than this range.
To increase the film forming rate, techniques for increasing the density of a silane gas or the input discharge power would be conceivable. However, the range of conditions which allows successful formation of a microcrystal silicon film is restricted; under the conditions out of that range, the crystal grain diameter of a film formed becomes too small, and reduction in crystal density prevents formation of a high-quality microcrystal silicon film.
The valence electron control of a microcrystal silicon film can be performed to obtain a film of p-type or n-type conductivity by adding an impurity during the film formation by using a doping gas of diborane, phosphine, or the like. It is an empirical fact that the addition of diborane, among those doping gases, particularly makes it difficult to effect microcrystallization.
The microcrystal silicon film is applied to the solar cell to form a p-type or n-type layer. To reduce the light absorption loss those layers are made as thin as about 10-50 nm at most. However, in forming such a thin microcrystal silicon film, the interaction with an undercoat material prevents sufficient microcrystallization.
For example, in forming a solar cell having a PIN junction, a heterojunction is formed by depositing a p-type layer of about 10 nm in thickness on an i-type amorphous silicon film. However, the deposition of a microcrystal film on an amorphous film causes lattice distortion, so that sufficient microcrystallization is not effected at the initial stage of the deposition and amorphous components become dominant in the corresponding region of a film formed. Thus, microcrystal silicon layers of solar cells formed according to the conventional techniques not necessarily have sufficient characteristics.
Although it is possible to produce a solar cell in which the entire PIN junction is made of microcrystal silicon, in this case the thickness of the i-type layer should be about 1,000 nm or preferably more than 1,000 nm due to the optical properties of the microcrystal silicon films. However, since the film forming rate of a microcrystalline silicon film is low, this type of configuration is not practical. For example, under film forming conditions for 0.03 nm/s, which is a typical film forming rate of a microcrystal silicon film, it takes more than 9 hours to deposit a 1,000-nm-thick film. This kind of process is extremely low in practicality.